Bug Report or Feature Request (mark with anx)

- [ ] bug report -> please search issues before submitting- [ x ] feature request

It would be nice to have full control of the connection to firebase. This would allow us to customize with:

• Options for firestore
• Emulators
• Lite vs full firestore

Repro steps

I'm using the approach described here:
https://github.com/angular/angularfire/blob/master/samples/modular/src/app/app.module.ts

And while it looks like it would be possible to use emulators by following this:#627

Desired functionality

Would like to use the modular approach from angularfire and have the library fetch the injected version instead.